Ancient Sunken Islands Found Off California

Sandy beaches are plentiful along the California coast, but not many are found
below the ocean. Scientists just added three to the list.

What were once volcanic islands off the coast of California now sit at the
bottom of sea.

With the help of a robotic submersible, a team of researchers captured images
of long sunk sandy beaches at depths exceeding 2,000 feet (610 meters). The
video images and rock samples taken from ocean bottom indicate that beaches
were associated with volcanoes that used to poke above the surface.

Map shows three seamounts
that appear to have been volcanic islands between 10 and 14 million years
ago. Credit: MBARI

The team from the Monterey Bay Aquarium Research Institute (MBARI), led by
volcanologist David Clague, guided the remotely operated vehicle Tiburon,
an unmanned submersible, and took the first video images of the Southern California
seamounts. Images revealed wave-cut rock surfaces, eroded beach sand, and rounded
pebbles - all evidence that these seamounts were once islands.

"They were probably islands for a million years or less and have been
submerged for about the last 10 to 14 million years," Clague told LiveScience,
adding that his team hasn't yet finished dating all of the samples hauled up
below.

Clague's team found the remnants of sandy beaches on the Rodriguez Seamount,
the San Juan Seamount and the Northeast Bank, each about 180 miles (300 kilometers)
off the coast of Los Angeles. The images were collected from eleven Tiburon
dives in October 2003 and April 2004 and were presented at the American Geophysical
Union 2004 Fall Meeting last month.

Seamounts are formed when undersea volcanoes erupt and lava flows build up
over time. The load of the volcano causes the earth around it to flex - similar
to how a board between two blocks would flex if you stand on it - and this force
causes the volcano to eventually sink until it settles.

This type of sinking behavior has been well studied in Hawaii, where some of
the islands have dropped by as much as 3,280 feet (1,000 meters) since they
were first formed.

Image from the ROV
Tiburon show deep-sea beach sand and lava that appears to have been sculpted
by waves -- along a shore now nearly a half-mile (680 meters) below the
surface.

Sometimes sandy deposits can be caused by the movements of glaciers during
eras of low sea level. Clague and his team say that the depths at which these
seamounts occur - each over a third of a mile below sea level - glacial activity
cannot be the explanation for these sandy deposits. Additionally, chemical and
mineralogical tests performed by the team on the many rock samples from the
seamounts suggest that the rocks had been exposed to air - this would only be
possible if the volcano had erupted above water.

"When a volcano erupts in the air, the lava actually oxidizes. Iron in
the rock combines with Oxygen in the air and makes reddish minerals," said
Clague, adding that some of the rocks his team has brought up were clearly sculpted
by waves and showed hydrodynamic shapes created from grinding sands.

Earlier explorations of a different group of seamounts off central California
did not reveal these telltale features. Although the central California seamounts
are larger, the water is much deeper there and these seamounts were probably
never exposed to air, even before they began to sink, explained Clague.

Seamounts are gaining more interest as biological hotspots, and Tiburon's
images revealed a rich array of colorful deep-sea marine-life around the seamounts,
including corals and sponges. Additionally, seamounts create swirling currents
that carry nutrients from the sea floor to shallower waters where fish can feed
on them.

These schools of fish draw the attention of larger predators, like tuna and
whales, as well as commercial fishermen.

Clague's team will now focus on why these seamounts are located where they
are and why they have these peculiar shapes.

Bjorn Carey is the science information officer at Stanford University. He has written and edited for various news outlets, including Live Science's Life's Little Mysteries, Space.com and Popular Science. When it comes to reporting on and explaining wacky science and weird news, Bjorn is your guy. He currently lives in the San Francisco Bay Area with his beautiful son and wife.